Typically, in radio communications systems, an attempt is made to use radio transmission techniques (including modulation, coding and antenna processing) that are suited to the channel conditions. By doing so, the radio channel is used efficiently.
To this end, radio transmission techniques are traditionally first chosen and then designed into wireless communication equipment based on an expected performance of a channel that the equipment will be using to communicate. The radio transmission techniques may also be chosen based on a need to deliver a guaranteed level of service, which may be, for example, defined by a data rate, an error rate or a combination thereof. As channel conditions are typically described by statistical functions that vary over time, the channel may change considerably during the use of the equipment. The traditional approach to choosing radio transmission techniques usually results in a conservative design that is reliable, but does not use the full potential channel capacity. Often the radio channel performs better than the worst-case conditions. Where wireless communication equipment is designed with the worst-case channel conditions in mind, the radio transmission technique chosen is often too conservative. That is, not as much data is sent through the channel as could be sent using modulation and/or coding techniques different from the chosen techniques. However, to maintain a guaranteed performance in the worst-case channel conditions, such conservative choices may be necessary.
In modern radio systems, the radio transmission techniques may be dynamically adapted to suit the channel conditions at the time of transmission. The equipment may be designed with a capability to adapt the radio transmission techniques quickly to respond to changes in the channel conditions. The response may include changes in the modulation techniques, coding techniques or antenna configurations for, say, beam tracking. Typically this adaptation involves a feedback control loop in which the channel conditions are measured at a remote receiver.
Channel conditions may, for instance, include a carrier to interference ratio or a data error rate. Measurements of these channel conditions may be signaled from the remote receiver to a transmitter so that the transmitter may adjust radio transmission parameters, such as power level, coding technique, modulation technique and antenna processing, to suit the signaled channel conditions. As the channel conditions vary over time, the radio transmission techniques may be adapted to suit the conditions reflected in the most recently received measurements. Thus, improved system performance may be achieved. System performance may be, for example, measured in terms of an amount of data throughput or a degree of interference with adjacent systems.
This adaptive communication technique is particularly suited to wireless Internet applications where the transmission of data may be delayed in time to await more favorable channel conditions. Advantageously, a constant user bit rate, which is a requirement of traditional radio systems, may not be a requirement of wireless Internet applications. It may also be important to improve the efficiency of frequency reuse (the simultaneous use of a frequency for two or more purposes) through, for example, antenna beam tracking.
One difficulty with this adaptive technique, however, is the requirement to estimate the current channel conditions. The adaptive technique provides the best performance when the channel conditions can be accurately determined. However, the channel conditions can change rapidly with time, particularly in a mobile communications environment, and the channel conditions may change significantly within a few milliseconds of being measured. The measurements may, thus, be of little benefit to the adaptive technique after the delay needed for the measurements to be signaled from the receiver to the transmitter. In a typical indoor office environment, measurements have shown that a channel may be completely decorrelated after about ten milliseconds (at a 900 MHz transmission frequency). Thus, the feedback control loop for the adaptive technique must be able to take measurements and provide the measurements to the transmitter within a few milliseconds for the information to be useful. Other studies have shown that a significant portion of the advantage of adaptive modulation and coding is lost if the channel information is old.
A traditional approach to (two way) radio system design places the two directions of transmission in different frequency channels. This separation of the transmission and reception frequency, known as Frequency Division Duplexing (FDD), is necessary to permit the radio apparatus to adequately separate the relatively strong local transmissions from the relatively (very) weak signals received from the other end. Unfortunately, because the receiver is receiving on a channel that is well separated, in frequency, from the channel used by a related transmitter, the channel conditions measured by the receiver may not be suitable for adapting the radio transmission techniques for the transmitter.